Human cytomegalovirus (HCMV), a herpes virus, is a ubiquitous human pathogen that infects over 60% of the adult population. It is a major cause of birth defects, a life-threatening opportunistic agent in immuno-suppressed people, and a possible cofactor in certain cancers. Using liquid chromatography-mass spectrometry-based metabolomics together with isotope tracers, we have discovered that HCMV profoundly up-regulates many host cell metabolic pathways, including glycolysis, the TCA cycle, and lipid biosynthesis. These metabolic changes are evocative of those occurring in cancer. We have compared HCMV and herpes simplex virus-1, and found that the metabolic effects of the latter virus, while also strong, are quite different from those of HCMV. This indicates that different related viruses encode distinct programs for host cell metabolic hijacking. Importantly, we have used siRNAs and drugs to show that successful HCMV replication depends on multiple metabolic enzymes. Inhibitors of acetyl-CoA carboxylase, elongases (ELOVLs) and acyl-CoA synthetases block the production of HCMV progeny. These enzymes are needed to produce very long chain fatty acids, and we have recently discovered that the envelope of HCMV virions is enriched about 10-fold in such fatty acid tails. Thus, HCMV's life cycle requires synthesis of specific lipid species. As inhibitors of lipid biosynthetic enzymes can be safe and well tolerated, the sensitivity of HCMV to host cell lipid production may reflect a therapeutically important metabolic vulnerability. The major metabolic effects of HCMV raise critical fundamental questions: How does HCMV alter host cell metabolism? Which specific lipids does HCMV require? How do these contribute to viral replication? We propose to address these questions using a multidisciplinary combination of virology, biochemistry and systems biology. Specifically, we will: (i) assess the program of lipid metabolic changes induced by HCMV and determine how drugs that block specific metabolic enzymes needed by the virus alter this program; (ii) investigate the mechanism by which HCMV alters metabolism; and (iii) determine how the metabolic changes support HCMV replication. These studies address an important and understudied area of HCMV biology, they will advance fundamental understanding of metabolic regulation, and they will lay the foundation for the discovery of novel anti-virals.